High-carbon hot-rolled steel sheet and method for manufacturing the same

ABSTRACT

A high-carbon hot-rolled steel sheet having a chemical composition containing, by mass %, C: 0.20% or more and 0.40% or less, Si: 0.10% or less, Mn: 0.50% or less, P: 0.03% or less, S: 0.010% or less, sol.A1: 0.10% or less, N: 0.0050% or less, B: 0.0005% or more and 0.0050% or less, and at least one of Sb, Sn, Bi, Ge, Te, and Se in an amount of 0.002% or more and 0.030% or less in total. The steel sheet has a microstructure including ferrite and cementite, in which the density of cementite in the ferrite grains is 0.08 pieces/m 2  or less. Additionally, the steel sheet has a hardness of 73 or less in terms of HRB and a total elongation of 39% or more.

TECHNICAL FIELD

The present invention relates to a high-carbon hot-rolled steel sheetand a method for manufacturing the steel sheet, and, in particular, to ahigh-carbon hot-rolled steel sheet excellent in terms of workability andhardenability to which B is added and which is highly effective forinhibiting nitrogen ingress in a surface layer thereof and a method formanufacturing the steel sheet.

BACKGROUND ART

Nowadays, automotive parts such as gears, transmission parts, and seatrecliner parts are manufactured by forming a hot-rolled steel sheet,which is carbon steel material for machine structural use prescribed inJIS G 4051, into desired shapes by using a cold forming method and byperforming a quenching treatment on the formed steel sheet in order toachieve a desired hardness. Therefore, a hot-rolled steel sheet, whichis a raw material for the parts, is required to have excellent coldworkability and hardenability, and various steel sheets have beenproposed to date.

For example, Patent Literature 1 discloses a medium-carbon steel sheetto be subjected to cold forming, the medium-carbon steel sheet having ahardness of 500 HV or more and 900 HV or less in the case where thesteel sheet is subjected to an induction hardening treatment in whichthe steel sheet is heated at an average heating rate of 100° C./s, thenheld at a temperature of 1000° C. for 10 seconds, and then rapidlycooled to room temperature at an average cooling rate of 200° C./s,having a chemical composition containing, by mass %, C: 0.30% to 0.60%,Si: 0.06% to 0.30%, Mn: 0.3% to 2.0%, P: 0.030% or less, S: 0.0075% orless, Al: 0.005% to 0.10%, N: 0.001% to 0.01%, Cr: 0.001% to 0.10%, and,optionally, one or more of Ni: 0.01% to 0.5%, Cu: 0.05% to 0.5%, Mo:0.01% to 0.5%, Nb: 0.01% to 0.5%, Ti: 0.001% to 0.05%, V: 0.01% to 0.5%,Ta: 0.01% to 0.5%, B: 0.001% to 0.01%, W: 0.01% to 0.5%, Sn: 0.003% to0.03%, Sb: 0.003% to 0.03%, and As: 0.003% to 0.03%, a microstructure,in which the average grain diameter d μm of carbides is 0.6 μm or less,in which the spheroidizing ratio P % of carbides is 70% or more and lessthan 90%, and in which the average grain diameter d pm of the carbidesand the spheroidizing ratio P % of the carbides satisfy the relationshipd≦0.04×P−2.6, and, optionally, a hardness of 120 HV or more and lessthan 170 HV before cold forming is performed. In addition, PatentLiterature 1 discloses a method for manufacturing such a medium-carbonsteel sheet to be subjected to cold forming in which steel having thechemical composition mentioned above is held at a temperature of 1050°C. to 1300° C., then subjected to hot rolling in which rolling isfinished at a temperature of 750° C. to 1000° C., then cooled to atemperature of 500° C. to 700° C. at a cooling rate of 20° C./s to 50°C./s, then cooled to a specified temperature at a cooling rate of 5°C./s to 30° C./s, then coiled, then held under specified conditions, andthen annealed at a temperature of 600° C. or higher and equal to orlower than the Ac_(t)-10° C.

In addition, Patent Literature 2 discloses a boron-added steel sheethaving a chemical composition containing, by mass %, C: 0.20% or moreand 0.45% or less, Si: 0.05% or more and 0.8% or less, Mn: 0.5% or moreand 2.0% or less, P: 0.001% or more and 0.04% or less, S: 0.0001% ormore and 0.006% or less, Al: 0.005% or more and 0.1% or less, Ti: 0.005%or more and 0.2% or less, B: 0.001% or more and 0.01% or less, N:0.0001% or more and 0.01% or less, and, optionally, one, two, or more ofCr: 0.05% or more and 0.35% or less, Ni: 0.01% or more and 1.0% or less,Cu: 0.05% or more and 0.5% or less, Mo: 0.01% or more and 1.0% or less,Nb: 0.01% or more and 0.5% or less, V: 0.01% or more and 0.5% or less,Ta: 0.01% or more and 0.5% or less, W: 0.01% or more and 0.5% or less,Sn: 0.003% or more and 0.03% or less, Sb: 0.003% or more and 0.03% orless, and As: 0.003% or more and 0.03% or less, in which an averageconcentration of a solid solution B in a region from the surface to adepth of 100 μm is 10 ppm or more. In addition, Patent Literature 2discloses that, in the case where annealing is performed in anatmosphere mainly containing nitrogen, since a phenomenon callednitrogen absorption occurs, B, which is an important chemical elementfrom the viewpoint of hardenability, combines with N in steel to form BNin an annealing process, which results in the effect of increasinghardenability through the use of B not being realized due to a decreasein the amount of a solid solution B. Patent Literature 2 discloses that,in order to achieve satisfactory hardenability, it is necessary tocontrol the concentration of a solid solution B in a region from thesurface to a depth of 100 μm to be 10 ppm or more, and that, therefore,it is important to suppress the influence of the atmosphere of a heatingprocess and an annealing process included in a manufacturing process. Inaddition, Patent Literature 2 discloses a method for manufacturing sucha boron-added steel sheet in which steel having the chemical compositionmentioned above is heated to a temperature of 1200° C. or lower, thensubjected to hot rolling with a finishing delivery temperature of 800°C. to 940° C., then cooled to a temperature of 650° C. or lower at acooling rate of 20° C./s or more, then cooled at a cooling rate of 20°C./s or less, then coiled at a temperature of 400° C. to 650° C., thenpickled, and then annealed at a temperature of 660° C. or higher andequal to or lower than the Ac₁ in an atmosphere in which hydrogenconcentration is 95% or more, the dew point in a temperature range lowerthan 400° C. is −20° C. or lower, and the dew point in a temperaturerange of 400° C. or higher is −40° C. or lower. In addition, PatentLiterature 2 discloses, for example, a method in which theabove-mentioned pickled steel sheet is further subjected to cold rollingand a method in which the above-mentioned annealed steel sheet issubjected to cold rolling, then further subjected to annealing in atemperature range from the Ac₁ to the Ac₁+50° C., and then subjected toslow cooling to a temperature of the Ac₁−30° C.

CITATION LIST Patent Literature

PTL 1: Japanese Patent No. 5048168

PTL 2: Japanese Patent No. 4782243

SUMMARY OF INVENTION Technical Problem

From the viewpoint of achieving good cold workability, a high-carbonhot-rolled steel sheet is required to have a comparatively low hardnessand a high elongation. For example, in the case where, for example,automotive parts, which have been manufactured by performing pluralprocesses such as hot forging, machining, and welding, are integrallymolded by performing cold press forming, the automotive parts arerequired to have properties such as a hardness of 73 or less in terms ofRockwell hardness HRB and a total elongation (El) of 39% or more. Inaddition, a high-carbon hot-rolled steel sheet having a comparativelylow hardness and a high elongation in order to achieve good workabilityas described above is required to have excellent hardenability suchthat, for example, the steel sheet has a Vickers hardness of HV440 ormore after water quenching has been performed.

In the case of the technique according to Patent Literature 1 where theaverage grain diameter of carbides is controlled to be 0.6 μm or less inorder to achieve quenching hardenability to be realized in an inductionhardening treatment which is performed at an average heating rate of100° C./s, since the average grain diameter of carbides is controlled tobe 0.6 μm or less in steel having a high C content of 0.3% to 0.6%,there is a tendency for strength to increase due to high density ofcarbides, which raises a risk of a decrease in workability. In addition,since, in the manufacturing method according to Patent Literature 1,two-step cooling control, in which cooling is performed to a temperatureof 500° C. to 700° C. at a cooling rate of 20° C./s to 50° C./s afterhot rolling has been performed, and then cooling is performed at acooling rate of 5° C./s to 30° C./s, is performed, there is a problem inthat it is difficult to control cooling.

Also, in the case of the technique according to Patent Literature 2where two-step cooling control, in which cooling is performed to atemperature of 650° C. or lower at a cooling rate of 20° C./s or moreafter hot rolling has been performed, and then cooling is performed at acooling rate of 20° C./s or lower, is performed, there is a problem inthat it is difficult to manage a cooling control. Moreover, in the caseof the technique according to Patent Literature 2, Mn is added in anamount of 0.5% or more in order to increase hardenability. Although Mnincreases hardenability, since there is an increase in the strength of ahot-rolled steel sheet through solid solution strengthening, there is anincrease in the hardness of the hot-rolled steel sheet.

On the other hand, B is known as chemical element that increaseshardenability when added in minute amounts, however, as described inPatent Literature 2, in the case where annealing is performed in anatmosphere containing mainly nitrogen, which is generally used as anatmospheric gas, there is a problem in that it is not possible torealize the effect of increasing hardenability caused by adding B due toa decrease in the amount of a solid solution B. Although, in PatentLiterature 2, such a problem is solved by performing annealing in anatmosphere containing 95% or more of hydrogen or in an atmosphere inwhich an inert gas such as Ar is used instead of hydrogen, there is anincrease in cdst in the case of a heat treatment in which such a gas isused. In addition, it is not clear whether or not it is possible toinhibit nitrogen absorption in an annealing process performed in anitrogen atmosphere only with this technique.

An object of the present invention is, in order to solve the problemsdescribed above, to provide a high-carbon hot-rolled steel sheet whoseraw material is B-added steel having a lower Mn content thanconventional steel, with which it is possible to stably achieveexcellent hardenability even if annealing is performed in a nitrogenatmosphere, and which has excellent workability corresponding to ahardness of 73 or less in terms of HRB and to a total elongation of 39%or more before a quenching treatment is performed and a method formanufacturing the steel sheet.

Solution to Problem

The present inventors diligently conducted investigations regarding therelationship between manufacturing conditions and workability andhardenability, in the case of a B-added high-carbon hot-rolled steelsheet having lower Mn content than conventional steel, that is, a Mncontent of 0.50% or less, and, as a result, obtained the followingknowledge.

i) The hardness and total elongation (hereafter, also simply referred toas elongation) of a high-carbon hot-rolled steel sheet before aquenching treatment is performed are strongly influenced by the densityof cementite in ferrite grains. In order to obtain a steel sheet havinga hardness of 73 or less in terms of HRP and a total elongation (El) of39% or _(more) it is necessary that the density of cementite in ferritegrains be 0.08 pieces/μm² or less.

ii) The density of cementite in ferrite grains is strongly influenced bythe finishing delivery temperature of finish rolling included in hotrolling and a cooling rate down to a temperature of 700° C. after finishrolling has been performed. In the case where the finishing deliverytemperature is excessively high or where the cooling rate is excessivelylow, since it is not possible to obtain a steel sheet having amicrostructure including pearlite and a specified volume fraction ofpro-eutectoid ferrite phase fraction after hot rolling has beenperformed, it is difficult to decrease the density of cementite afterspheroidizing annealing has been performed.

iii) By adding at least one of Sb, Sn, Bi, Ge, Te, and Se to steel,since it is possible to prevent nitrogen ingress even if annealing isperformed in a nitrogen atmosphere, it is possible to achieve highhardenability by inhibiting a decrease in the amount of a solid solutionB.

The present invention has been completed on the basis of such knowledge,and the subjective matter of the present invention is as follows.

[1] A high-carbon hot-rolled steel sheet having a chemical compositioncontaining, by mass %, C: 0.20% or more and 0.40% or less, Si: 0.10% orless, Mn: 0.50% or less, P: 0.03% or less, S: 0.010% or less, sol.Al:0.10% or less, N: 0.0050% or less, B: 0.0005% or more and 0.0050% orless, one or more of Sb, Sn, Bi, Ge, Te, and Se in an amount of 0.002%or more and 0.030% or less in total, and the balance being Fe andinevitable impurities, in which the proportion of the content of a solidsolution B to the content of B is 70% or more, a microstructureincluding ferrite and cementite, in which the density of cementite inthe ferrite grains is 0.08 pieces/μm² or less, a hardness of 73 or lessin terms of HRB, and a total elongation of 39% or more.

[2] The high-carbon hot-rolled steel sheet according to item [1] above,the steel sheet having the chemical composition further containing, bymass %, one or more of Ni, Cr, and Mo in an amount of 0.50% or less intotal.

[3] The high-carbon hot-rolled steel sheet according to item [1] or [2]above, the steel sheet having the microstructure including ferrite andcementite, in which the average grain diameter of all the cementite is0.60 μm or more and 1.00 μm or less, and in which the average graindiameter of cementite in ferrite grains is 0.40 μm or more.

[4] A method for manufacturing a high-carbon hot-rolled steel sheet, themethod including performing hot rough rolling on steel having a chemicalcomposition containing, by mass %, C: 0.20% or more and 0.40% or less,Si: 0.10% or less, Mn: 0.50% or less, 2: 0.03% or less, S: 0.010% orless, sol.Al: 0.10% or less, N: 0.0050% or less, B: 0.0005% or more and0.0050% or less, one or more of Sb, Sn, Bi, Ge, Te, and Se in an amountof 0.002% or more and 0.030% or less in total, and the balance being Feand inevitable impurities, then performing finish rolling with afinishing delivery temperature equal to or higher than the Ar_(a)transformation temperature and 870° C. or lower, then cooling thehot-rolled steel sheet to a temperature of 700° C. at an average coolingrate of 25° C./s or more and 150° C./s or less, then coiling the cooledsteel sheet at a coiling temperature of 500° C. or higher and 700° C. orlower in order to obtain a steel sheet having a microstructure includingpearlite and, in terms of volume fraction, 5% or more of pro-eutectoidferrite, and then annealing the steel sheet at a temperature equal to orlower than the Ac₁ transformation temperature.

[5] The method for manufacturing a high-carbon hot-rolled steel sheetaccording to item [4] above, the steel having the chemical compositionfurther containing, by mass %, one or more of Ni, Cr, and Mo in anamount of 0.50% or less in total.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, it is possible to manufacture ahigh-carbon hot-rolled steel sheet excellent in terms of hardenabilityand workability. The high-carbon hot-rolled steel sheet according to thepresent invention can preferably be used for automotive parts such asgears, transmission parts, and seat recliner parts whose raw materialsteel sheet is required to have satisfactory cold workability.

Description of Embodiments

Hereafter, the high-carbon hot-rolled steel sheet and the method formanufacturing the steel sheet according to the present invention will bedescribed in detail. Here, “%”, which is the unit of the content of aconstituent chemical element, refers to “mass %”, unless otherwisenoted.

1) Chemical composition

C: 0.20% or more and 0.40% or less

C is a chemical element which is important for achieving strength afterquenching has been performed. In the case where the C content is lessthan 0.20%, it is not possible to achieve the desired hardness byperforming a heat treatment after a part has been formed, or,specifically, it is not possible to achieve a hardness of HV440 or moreafter water quenching has been performed. Therefore, it is necessarythat the C content be 0.20% or more. On the other hand, in the casewhere the C content is more than 0.40%, since there is an increase inthe hardness of a steel sheet, there is a decrease in cold workability.Therefore, the C content is set to be 0.40% or less. It is preferablethat the C content be 0.26% or more in order to achieve a high quenchedhardness. It is more preferable that the C content be 0.32% or more,because it is possible to stably achieve a hardness of HV440 or moreafter water quenching has been performed.

Si: 0.10% or less

Si is a chemical element which increases strength through solid solutionstrengthening. Since hardness increases with increasing Si content,there is a decrease in cold workability. Therefore, the Si content isset to be 0.10% or less, preferably 0.05% or less, or more preferably0.03% or less. Although it is preferable that the Si content be as smallas possible because Si decreases cold workability, since there is anincrease in refining costs in the case where the Si content isexcessively decreased, it is preferable that the Si content be 0.005% ormore.

Mn: 0.50% or less

Although Mn is a chemical element which increases hardenability, but, onthe other hand, Mn is also a chemical element which increases strengththrough solid solution strengthening. In the case where the Mn contentis more than 0.50%, there is a decrease in cold workability due to anexcessive increase in the hardness of a steel sheet. In addition, sincea band structure grows due to the segregation of Mn and a non-uniformmicrostructure is formed, there is a tendency for a variation inhardness and elongation to increase. Therefore, the Mn content is set tobe 0.50% or less, preferably 0.45% or less, or more preferably 0.40% orless. Here, although there is no particular limitation on the lowerlimit of the Mn content, it is preferable that the Mn content be 0.20%or more in order to achieve the specified quenched hardness by allowingall the C in a steel sheet to form a solid solution in a heating processfor a quenching treatment as a result of inhibiting the precipitation ofgraphite.

P: 0.03% or less

P is a chemical element which increases strength through solid solutionstrengthening. In the case where the P content is more than 0.03%, thereis a decrease in cold workability due to an excessive increase in thehardness of a steel sheet. In addition, since there is a decrease in thestrength of grain boundaries, there is a decrease in toughness afterquenching has been performed. Therefore, the P content is set to be0.03% or less. It is preferable that the P content be 0.02% or less inorder to achieve excellent toughness after quenching has been performed.Although it is preferable that the P content be as small as possiblebecause P decreases cold workability and toughness after quenching hasbeen performed, since there is an increase in refining costs in the casewhere the P content is decreased more than necessary, it is preferablethat the P content be 0.005% or more.

S: 0.010% or less

Since S forms sulfides and decreases the cold workability of ahigh-carbon hot-rolled steel sheet and toughness after quenching hasbeen performed, S is a chemical element whose content should bedecreased. In the case where the S content is more than 0.010%, there isa significant decrease in the cold workability of a high-carbonhot-rolled steel sheet and toughness after quenching has been performed.Therefore, the S content is set to be 0.010% or less. It is preferablethat the S content be 0.005% or less in order to achieve excellent coldworkability and excellent toughness after quenching has been performed.Although it is preferable that the S content be as small as possiblebecause S decreases cold workability and toughness after quenching hasbeen performed, since there is an increase in refining costs in the casewhere the S content is decreased more than necessary, it is preferablethat the S content be 0.0005% or more.

sol.Al: 0.10% or less

In the case where the sol.Al content is more than 0.10%, since there isan excessive decrease in austenite grain diameter due to the formationof AlN in a heating process for a quenching treatment, a microstructureincluding ferrite and martensite is formed as a result of promoting theformation of a ferrite phase in a cooling process, which results in adecrease in hardness after quenching has been performed. Therefore, thesol.Al content is set to be 0.10% or less, or preferably 0.06% or less.Here, Al is effective for deoxidation, and it is preferable that thesol.Al content be 0.005% or more in order to sufficiently performdeoxidation.

N: 0.0050% or less

In the case where the N content is more than 0.0050%, since an excessiveamount of BN is formed, there is a decrease in the amount of a solidsolution B. In addition, since BN and AlN are formed in amounts largerthan necessary, there is an excessive decrease in austenite graindiameter in a heating process for a quenching treatment, the formationof a ferrite phase is promoted in a cooling process, which results in adecrease in hardness after quenching has been performed. Therefore, theN content is set to be 0.0050% or less, or preferably 0.0045% or less.Here, although there is no particular limitation on the lower limit ofthe N content, N forms BN and AlN as described above. In the case whereappropriate amounts of BN and AlN are formed, since such nitridessuitably inhibit an increase in austenite grain diameter in a heatingprocess for a quenching treatment, there is an increase in toughnessafter quenching has been performed. Therefore, it is preferable that theN content be 0.0005% or more.

B: 0.0005% or more and 0.0050% or less

B is an important chemical element which increases hardenability. Underthe condition regarding the cooling rate after finish rolling has beenperformed in hot rolling according to the present invention, in the casewhere the B content is less than 0.0005%, since there is an insufficientamount of a solid solution B, which delays ferrite transformation, it isnot possible to realize sufficient effect of increasing hardenability.Therefore, it is necessary that the B content be 0.0005% or more, orpreferably 0.0010% or more. On the other hand, in the case where the Bcontent is more than 0.0050%, the recrystallization of austenite afterfinish rolling has been performed is delayed. As a result, since therolled texture of a hot-rolled steel sheet grows, there is an increasein the in-plane anisotropy of the mechanical properties of a steel sheetafter annealing has been performed. Therefore, since earing tends tooccur and there is a decrease in roundness when drawing is performed,problems tend to occur when forming is performed. Therefore, it isnecessary that the B content be 0.0050% or less. It is preferable thatthe B content be 0.0035% or less from the viewpoint of increasinghardenability and of decreasing anisotropy. Therefore, the B content isset to be 0.0005% or more and 0.0050% or less, or preferably 0.0010% ormore and 0.0035% or less.

The proportion of the content of a solid solution B to the content of B:70% or more

In the present invention, in addition to the optimization of the Bcontent described above, the control of the amount of a solid solutionB, which contributes to an increase in hardenability, is important. Inthe case where the proportion of the amount of B present in a solidsolution state to the amount of B contained in a steel sheet is 70% ormore, that is, in the case where the proportion of the content of asolid solution B to the total content of B (B content) in a steel sheetis 70% or more, it is possible to achieve excellent hardenabilitytargeted in the present invention. Therefore, the proportion of thecontent of a solid solution B to the content of B is set to be 70% ormore, or preferably 75% or more. Here, “the proportion of the content ofa solid solution B to the content of B” refers to {(content of a solidsolution B (mass %))/(total B content (mass %))}×100 (%).

One or more of Sb, Sn, Bi, Ge, Te, and Se: 0.002% or more and 0.030% orless in total

Sb, Sn, Bi, Ge, Te, and Se are all chemical elements which are effectivefor inhibiting nitrogen ingress through the surface of a steel sheet,and it is necessary that one or more of Sb, Sn, Bi, Ge, Te, and Se beadded in the present invention. In addition, in the case where the totalcontent of these chemical elements is less than 0.002%, sufficienteffect of inhibiting nitrogen ingress is not realized. Therefore, one ormore of Sb, Sn, Bi, Ge, Te, and Se is added in an amount of 0.002% ormore in total, or preferably 0.005% or more in total. On the other hand,in the case where the total content of these chemical elements is morethan 0.030%, the effect of inhibiting nitrogen ingress becomessaturated. In addition, since these chemical elements tend to besegregated at grain boundaries, grain boundary embrittlement may occurin the case where the total content of these chemical elements is morethan 0.030%. Therefore, in the present invention, one or more of Sb, Sn,Bi, Ge, Te, and Se is added in an amount of 0.030% or less in total, orpreferably 0.020% or less.

As described above, by controlling the N content to be 0.0050% or less,and by adding one or more of Sb, Sn, Bi, Ge, Te, and Se in an amount of0.002% or more and 0.030% or less in total, since it is possible toinhibit an increase in nitrogen concentration in the surface layer of asteel sheet by inhibiting nitrogen ingress through the surface of thesteel sheet even in the case where annealing is performed in a nitrogenatmosphere, it is possible to control the difference between an averagenitrogen concentration in a region from the surface to a depth of 150 μmin the thickness direction of the steel sheet and an average nitrogenconcentration in the whole steel sheet to be 30 mass ppm or less. Inaddition, since it is possible to inhibit nitrogen ingress as describedabove, it is possible to control the proportion of the content of asolid solution B to the content of B to be 70% or more in a steel sheetafter annealing has been performed even if annealing is performed in anitrogen atmosphere.

In the case where the difference between an average nitrogenconcentration in a region from the surface to a depth of 150 μm in thethickness direction of the steel sheet and an average nitrogenconcentration in the whole steel sheet is more than 30 mass ppm, thereis an increase in the difference between the amounts of BN and AINformed in the surface layer, of the steel sheet and the amounts of BNand AlN formed in the vicinity of the central portion in the thicknessdirection of the steel sheet. In this case, there is a problem such asone in that it is not possible to achieve uniform hardness distributionafter a quenching treatment has been performed. Therefore, it isnecessary to suppress the difference between an average nitrogenconcentration in a region from the surface to a depth of 150 μm thethickness direction of the steel sheet and an average nitrogenconcentration in the whole steel sheet to be 30 mass ppm or less.

Although remainder other than those above is Fe and inevitableimpurities, one or more of Ni, Cr, and Mo may be added in order tofurther increase hardenability. In order to realize such an effect, itis preferable that one or more of Ni, Cr, and Mo be added and that thetotal content of these chemical elements be 0.01% or more. On the otherhand, since these chemical elements are expensive, in the case where oneor more of Ni, Cr, and Mo are added, it is necessary that the totalcontent of these chemical elements be 0.50% or less, or preferably 0.20%or less.

2) Microstructure

In the present invention, in order to increase cold workability, it isnecessary that a microstructure including ferrite and cementite beformed by performing annealing (spheroidizing annealing), in whichspheroidal cementite is formed, after hot rolling has been performed.Here, “spheroidal” refers to a case where the proportion of the amountof cementite having an aspect ratio (the length of major axis/the lengthof minor axis) of 3 or less to the total amount of cementite is 90% ormore in terms of volume fraction. In particular, in order to achieve aRockwell hardness of 73 or less in terms of HRB and a total elongationof 39% or more, it is necessary that the density of cementite in ferritegrains be 0.08 pieces/μm² or less. Hereinafter, “the density ofcementite” is also referred to as “the number density of cementitegrains”.

Number density of cementite grains in ferrite grains: 0.08 pieces/μm² orless

The steel sheet according to the present invention has a microstructureincluding ferrite and cementite. In the case where the number density ofcementite grains in ferrite grains is high, there is an increase inhardness due to dispersion strengthening and there is a decrease inelongation. In order to control hardness to be equal to or less than thespecified value and in order to control elongation to be equal to ormore than the specified value, it is necessary that the number densityof cementite grains in ferrite grains be 0.08 pieces/μm² or less,preferably 0.07 pieces/μm² or less, or more preferably 0.06 pieces/μm²or less. Since the length of the major axis of cementite grains inferrite grains is about 0.15 μm to 1.8 μm, the sizes of cementite grainsslightly contributes to precipitation strengthening of a steel sheet.Therefore, it is possible to decrease strength by decreasing the numberdensity of cementite grains in ferrite grains. Since cementite grainsexisting at ferrite grain boundaries scarcely contribute to dispersionstrengthening, the number density of cementite grains in ferrite grainsis set to be 0.08 pieces/μm² or less. Here, it is acceptable thatremaining microstructures such as pearlite other than ferrite andcementite described above be inevitably formed in the case where thetotal volume fraction of the remaining microstructures be about 5% orless, because the effects of the present invention are not decreased.

Average grain diameter of all the cementite: 0.60 μm or more and 1.00 μmor less and average grain diameter of cementite in ferrite grains: 0.40μm or more

In the case of a steel sheet in which the average grain diameter ofcementite in ferrite grains is less than 0.40 μm, since there is anincrease in the number density of cementite grains in ferrite grains,there is a case where there is an increase in the hardness of the steelsheet after annealing has been performed. In order to control hardnessto be equal to or less than the desired value, it is preferable that theaverage grain diameter of cementite in ferrite grains be 0.40 μm ormore, or more preferably 0.45 μm or more.

Since the grain diameter of cementite at ferrite grain boundaries ismore likely to increase than that of cementite in ferrite grains, it isnecessary that the average grain diameter of all the cementite be 0.60μm or more, or preferably 0.65 μm or more, in order to control theaverage grain diameter of cementite in ferrite grains to be 0.40 μm ormore. On the other hand, in the case where the average grain diameter ofall the cementite is more than 1.00 μm, since cementite is notcompletely dissolved in a short-time heating such as heating for aninduction hardening treatment, there is a case where it is not possibleto control hardness to be equal to or less than the desired value.Therefore, it is preferable that the average grain diameter of all thecementite be 1.00 μm or less, or more preferably 0.95 μm or less.Regarding the average grain diameter of cementite described above, it ispossible to determine the average grain diameter of all the cementiteand the average grain diameter of cementite in ferrite grains byobserving the microstructure by using a SEM and by determining thelengths of the major axis and minor axis of cementite grains.

Here, in the case where the grain diameter of ferrite is excessivelylarge, although there is a decrease in hardness, since there is a casewhere the effect of increasing elongation becomes saturated, it ispreferable that the average grain diameter of ferrite be 12 μm or less,or more preferably 9 μm or less, in the microstructure including ferriteand cementite described above. On the other hand, in the case where theaverage grain diameter of ferrite is less than 6 μm, there is a casewhere there is an increase in the hardness of a steel sheet. Therefore,it is preferable that the average grain diameter of ferrite be 6 μm ormore. It is possible to determine the grain diameter of ferritedescribed above by observing the microstructure by using a SEM.

3) Mechanical Properties

In the case of the steel sheet according to the present invention, sinceautomotive parts such as gears, transmission parts, and seat reclinerparts are formed by performing cold press forming, excellent workabilityis required. In addition, it is necessary to provide abrasion resistanceto the parts by increasing hardness by performing a quenching treatment.Therefore, in addition to increasing hardenability, it is necessary thatthe hardness of a steel sheet is decreased to 73 or less in terms of HRBand total elongation (El) of a steel sheet is increased to 39% or more.Although it is preferable that the hardness of a steel sheet be as lowas possible from the viewpoint of workability, since some parts arepartially subjected quenching, the strength of a raw material steelsheet may influence fatigue characteristics. Therefore, it is preferablethat the hardness of a steel sheet be more than 60 in terms of HRB.Here, it is possible to determine hardness in terms HRB described aboveby using a Rockwell hardness meter (B scale). In addition, it ispossible to determine total elongation by performing a tensile test at atensile speed of 10 mm/min on a JIS No. 5 tensile test piece which hasbeen taken in a direction (L-direction) at an angle of 0° to the rollingdirection by using tensile test machine AG-10TB AG/XR produced bySHIMADZU CORPORATION and by butting the pieces of a broken sample.

4) Manufacturing Condition

The high-carbon hot-rolled steel sheet according to the presentinvention is manufactured by using raw material steel having thechemical composition described above, by performing hot rollingincluding performing hot rough rolling and then performing hot finishrolling with a finishing delivery temperature equal to or higher thanthe Ar₃ transformation temperature and 870° C. or lower in order toobtain a desired thickness, by then cooling the hot-rolled steel sheetto a temperature of 700° C. at an average cooling rate of 25° C./s ormore and 150° C./s or less, by then coiling the cooled steel sheet at acoiling temperature of 500° C. or higher and 700° C. or lower in orderto obtain a steel sheet having a microstructure including pearlite and,in terms of volume fraction, 5% or more of pro-eutectoid ferrite, and bythen performing spheroidizing annealing on the steel sheet at atemperature equal to or lower than the Ac₁ transformation temperature.Here, it is preferable that the rolling reduction of finish rolling be85% or more.

Hereafter, the reasons for limitations on the method for manufacturing ahigh-carbon hot-rolled steel sheet according to the present inventionwill be described.

Finishing delivery temperature: equal to or higher than the Ar₃transformation temperature and 870° C. or lower

In order to control the number density of cementite grains in ferritegrains to be 0.08 pieces/μm² or less after annealing has been performed,it is necessary to perform spheroidizing annealing on a hot-rolled steelsheet having a microstructure including pearlite and, in terms of volumefraction, 5% or more of pro-eutectoid ferrite. In the case where thefinishing delivery temperature is higher than 870° C. in hot rolling inwhich finish rolling is performed after hot rough rolling has beenperformed, since there is a decrease in the proportion of pro-eutectoidferrite, it is not possible to achieve the specified number density ofcementite grains after spheroidizing annealing has been performed. Andthere is a tendency for cementite grain diameter and ferrite graindiameter to increase after spheroidizing annealing has been performed.Therefore, the finishing delivery temperature is set to be 870° C. orlower. In order to sufficiently increase the proportion of pro-eutectoidferrite, it is preferable that the finishing delivery temperature be850° C. or lower. On the other hand, in the case where the finishingdelivery temperature is lower than the Ar₁ transformation temperature,since ferrite grains having a large grain diameter are formed after hotrolling or annealing has been performed, there is a significant decreasein elongation. Therefore, the finishing delivery temperature is set tobe equal to or higher than the Ar₃ transformation temperature, orpreferably 820° C. or higher. Here, “finishing delivery temperature”refers to the surface temperature of a steel sheet.

Average cooling rate from finishing delivery temperature to 700° C.: 25°C./s or more and 150° C./s or less

In order to control the number density of cementite grains in ferritegrains to be 0.08 pieces/μm² or less after annealing has been performed,it is necessary to perform spheroidizing annealing on a hot-rolled steelsheet having a microstructure including pearlite and, in terms of volumefraction, 5% or more of pro-eutectoid ferrite. Since a temperature rangedown to a temperature of 700° C. after finish rolling included in hotrolling has been performed is a temperature range in which ferritetransformation start temperature and pearlite transformation starttemperature exist, the cooling rate from the finishing deliverytemperature to 700° C. is an important factor in order to control apro-eutectoid ferrite phase fraction in a steel sheet after hot rollinghas been performed to be 5% or more in terms of volume fraction. In thecase where the average cooling rate in a temperature range from thefinishing delivery temperature to 700° C. is less than 25° C./s, sinceferrite transformation is less likely to progress in a short time, whichresults in an increase in pearlite phase fraction more than necessary,it is not possible to form, in terms of volume fraction, 5% or more ofpro-eutectoid ferrite. In addition, since pearlite having a large graindiameter is formed, it is difficult to form the desired steel sheetmicrostructure after spheroidizing annealing has been performed.Therefore, the average cooling rate in a temperature range down to atemperature of 700° C. after finish rolling has been performed is set tobe 25° C./s or more. In addition, since it is preferable that thepro-eutectoid ferrite phase fraction be 10% or more in terms of volumefraction in order to control the number density of cementite grains inferrite grains to be 0.07 pieces/μm² or less after spheroidizingannealing has been performed, it is preferable that the average coolingrate be 30° C./s or more, or more preferably 40° C./s or more, in thiscase. On the other hand, in the case where the average cooling rate ismore than 150° C./s, it is difficult to form pro-eutectoid ferrite.Therefore, the average cooling rate down to a temperature of 700° C.after finish rolling has been performed is set to be 150° C./s or less,preferably 120° C./s or less, or more preferably 100° C./s or less.Here, this “temperature” refers to the surface temperature of a steelsheet.

Coiling temperature: 500° C. or higher and 700° C. or lower

The steel sheet which has been subjected to finish rolling is wound in acoil shape at a coiling temperature of 500° C. or higher and 700° C. orlower after cooling has been performed as described above. It is notpreferable that the coiling temperature be higher than 700° C., becauseit is not possible to form the desired steel sheet microstructure afterannealing has been performed due to an increase in the grain diameter ofthe microstructure of a hot-rolled steel sheet, and because, from theviewpoint of operational efficiency, there is a case where coil deformsunder its own weight due to an excessive decrease in the strength of asteel sheet when the steel sheet is wound in a coil shape. Therefore,the coiling temperature is set to be 700° C. or lower, or preferably650° C. or lower. On the other hand, in the case where the coilingtemperature is lower than 500° C., since there is an increase in thehardness of a steel sheet due to a decrease in the grain diameter of thesteel sheet microstructure, there is a decrease in workability due to adecrease in elongation. Therefore, the coiling temperature is set to be500° C. or higher, or preferably 550° C. or higher. Here, “coilingtemperature” refers to the surface temperature of a steel sheet.

Steel sheet microstructure after hot rolling has been performed:including pearlite and, in terms of volume fraction, 5% or more ofpro-eutectoid ferrite

In the present invention, after spheroidizing annealing has beenperformed as described below, a steel sheet having a microstructurewhich includes ferrite and cementite and in which the number density ofcementite grains in the ferrite grains is 0.08 pieces/μm² or less isobtained. The microstructure after spheroidizing annealing has beenperformed is strongly influenced by the steel sheet microstructure afterhot rolling has been performed. By forming a steel sheet microstructureincluding pearlite and, in terms of volume fraction, 5% or more ofpro-eutectoid ferrite after hot rolling has been performed, since it ispossible to form the desired microstructure after spheroidizingannealing has been performed, it is possible to obtain steel having highworkability. In addition, in the case of a steel sheet having amicrostructure which does not include pearlite or in which apro-eutectoid ferrite phase fraction is less than 5% in terms of volumefraction, since it is not possible to achieve the specified numberdensity of cementite grains after spheroidizing annealing has beenperformed at a temperature equal to or lower than the Ac₁ transformationtemperature, there is an increase in the strength of a steel sheet.Therefore, the microstructure of a steel sheet (hot-rolled steel sheet)obtained by performing hot rolling, cooling, and coiling under theconditions described above is a microstructure including pearlite and,in terms of volume fraction, 5% or more of pro-eutectoid ferrite, orpreferably, pearlite and, in terms of volume fraction, 10% or more ofpro-eutectoid ferrite. Here, in order to achieve a higher level ofuniformity in a microstructure after annealing has been performed, it ispreferable that the pro-eutectoid ferrite phase fraction be 50% or lessin terms of volume fraction.

Annealing temperature: equal to or lower than the Ac₁ transformationtemperature

The hot-rolled steel sheet obtained as described above is subjected toannealing (spheroidizing annealing). In the case where the annealingtemperature is higher than the Ac₁ transformation temperature, sinceaustenite is formed, a pearlite structure having a large grain diameteris formed in a cooling process following the annealing process, whichresults in a non-uniform microstructure being formed. Therefore, theannealing temperature is set to be equal to or lower than the Ac₁transformation temperature. Here, although there is no particularlimitation on the lower limit of the annealing temperature, it ispreferable that the annealing temperature be 600° C. or higher, or morepreferably 700° C. or higher, in order to control the number density ofcementite grains in ferrite grains to be the desired value. Here, as anatmospheric gas, any of nitrogen, hydrogen, and a mixed gas of nitrogenand hydrogen may be used, and, although, it is preferable to use suchgases, Ar may also be used without any particular limitation. Inaddition, it is preferable the annealing time be 0.5 hours or more and40 hours or less. By controlling the annealing time to be 0.5 hours ormore, since it is possible to stably form the desired microstructure, itis possible to control the hardness of a steel sheet to be equal to orlower than the desired value, and it is possible to control elongationto be equal to or more than the desired value. Therefore, it ispreferable the annealing time be 0.5 hours or more, or more preferably 8hours or more. In addition, in the case where the annealing time is morethan 40 hours, there is a decrease in productivity, and there istendency for manufacturing costs to excessively increase. Therefore, itis preferable that the annealing time be 40 hours or less. Here,“annealing temperature” refers to the surface temperature of a steelsheet. In addition, “annealing time” refers to a period of time duringwhich the specified temperature is maintained.

Here, in order to prepare the molten material of the high-carbon steelaccording to the present invention, any of a converter and an electricfurnace may be used. In addition, the molten material of the high-carbonsteel prepared as described above is made into a slab by using an ingotcasting-slabbing method or a continuous casting method. The slab isusually heated and then subjected to hot rolling. Here, in the case of aslab manufactured by using a continuous casting method, hot directrolling, which is performed on the slab in the cast state or after heatretention has been performed in order to inhibit a fall in temperature,may be performed. In addition, in the case where slab is subjected tohot rolling after heating has been performed, it is preferable that theslab heating temperature be 1280° C. or lower in order to inhibit adeterioration in surface quality due to scale. In hot rolling, in orderto perform finish rolling at a specified temperature, the material to berolled may be heated by using a heating means such as a sheet bar heaterin a hot rolling process.

EXAMPLE 1

By preparing molten steels having the chemical compositionscorresponding to steel codes A through H given in Table 1, and by thenperforming finish rolling, cooling, and coiling under the hot rollingconditions given in Table 2, hot rolled steel sheets were obtained.Here, the cooling rates given in Table 2 were the average cooling ratesdown to a temperature of 700° C. after finish rolling has beenperformed. Subsequently, by performing pickling, and by performingannealing (spheroidizing annealing) in a nitrogen atmosphere(atmospheric gas: nitrogen) under the annealing conditions given inTable 2, hot-rolled steel sheets (hot-rolled and annealed steel sheets)having a thickness of 4.0 mm and a width of 1000 mm were manufactured.The hardness, elongation, and microstructure of the hot-rolled andannealed steel sheets manufactured as described above were investigated.In addition, the microstructures of the hot-rolled steel sheets beforeannealing was performed were also investigated. The results are given inTable 2. Here, the Ar₃ transformation temperatures and the Ac₁transformation temperatures given in Table 1 were derived by using aformaster.

Hardness (HRB) of Hot-Rolled and Annealed Steel Sheet

By taking a sample from the central portion in the width direction ofthe annealed steel sheet, and by determining hardness at five points byusing a Rockwell hardness meter (B scale), an average value was derived.

Total elongation (El) of Hot-Rolled and Annealed Steel Sheet

By performing a tensile test at a tensile speed of 10 mm/min on a JISNo. 5 tensile test piece which had been taken from the annealed steelsheet in a direction (L-direction) at an angle of 0° to the rollingdirection by using tensile test machine AG-10TB AG/XR produced bySHIMADZU CORPORATION, and by butting the pieces of a broken sample,elongation (total elongation) was derived.

Microstructure

By observing the microstructure of the hot-rolled steel sheet beforeannealing was performed (the microstructure of the hot-rolled steelsheet) by using a SEM, the kinds of the microstructures were identified,and a pro-eutectoid ferrite phase fraction was derived. Bydistinguishing the area of ferrite from the area of other phases, and byderiving the proportion of the area of ferrite in order to deriving anarea fraction, the volume fraction of pro-eutectoid ferrite wasdetermined as the obtained area fraction thereof. Here, it was confirmedthat pearlite existed in the hot-rolled steel sheet before annealing wasperformed given in Table 2 in the SEM observation described above.

The microstructure of the hot-rolled steel sheet after annealing hadbeen performed (the microstructure of the hot-rolled and annealed steelsheet) was observed by using microstructure photographs which werecaptured by using a scanning electron microscope at a magnification of3000 times at five positions located at a depth of ¼ in the thicknessdirection of a sample which had been prepared by taking the sample fromthe central portion in the width direction of the steel sheet, byperforming cutting and polishing, and by performing nital etching. Byidentifying the kinds of the microstructures of the sample, by countingthe number of cementite grains which did not exist at grain boundariesand which had a major axis of 0.15 μm or more, and by dividing thenumber by the area of the fields of view of the photographs, the densityof cementite in ferrite grains (the number density of cementite grainsin ferrite grains) was derived. By determining the lengths of the majoraxis and minor axis of each of the cementite grains by using themicrostructure photographs described above, the average grain diameterof all the cementite and the average grain diameter of cementite ingrains were derived. Ferrite grain diameter was derived by determininggrain size by using the microstructure photograph described above, andthen average ferrite grain diameter was calculated.

In addition, with respect to the steel sheet after annealing had beenperformed (hot-rolled and annealed steel sheet), the difference betweenN content in a region from the surface to a depth of 150 μl of thesurface layer and the average N content of the steel sheet and theproportion of the content of a solid solution B to the content of B werederived by using the following methods. The results are given in Table2.

Difference Between Average N Content within 150 μm of the Surface Layerand the Average N Content of the Steel Sheet

With respect to a sample taken from the central portion in the widthdirection of the steel sheet after annealing had been performed, averageN content within 150 μm of the surface layer and the average N contentof the steel sheet were determined, and then the difference between theaverage N content within 150 μm of the surface layer and the average Ncontent of the steel sheet was derived. Here, “average N content within150 μm of the surface layer” refers to average N content in a regionfrom the surface of the steel sheet to a depth of 150 μm in thethickness direction. In addition, the average N content within 150 μm ofthe surface layer was derived by using the following method. That is, bystarting machining from the surface of a taken sample steel sheet, andby machining the steel sheet to a depth of 150 μm from the surfacethereof, the produced cutting chips were collected as samples. Theaverage N content within 150 μm of the surface layer was defined as theN content of the samples. The average N content within 150 μm of thesurface layer and the average N content of the steel sheet weredetermined by using an inert gas fusion-thermal conductivity method. Acase where the difference between the average N content within 150 μm ofthe surface layer (N content in a region from the surface to a depth of150 μm from the surface) and the average N content of the steel sheet (Ncontent in the steel) determined as described above was 30 mass ppm orless may be judged as a case where nitrogen ingress was inhibited.

Proportion of the Content of a Solid Solution B to the Content of B

A sample was taken from the central portion in the width direction ofthe steel sheet after annealing had been performed. By extracting BN insteel by using 10 vol. %-Br-methanol, by subtracting the content of Bwhich was precipitated in the form of BN from the total content of B insteel, the amount of a solid solution B was derived. The proportion ofthe content of a solid solution B to the total content of B (B content)in steel was calculated to be equal to {(content of a solid solution B(mass %))/(total B content (mass %))}×100 (%). A case where thisproportion was 70 (%) or more may be judged as a case where a decreasein the content of a solid solution B was inhibited.

Hardness (Quenched Hardness) of a Steel Sheet After Quenching has BeenPerformed

In addition, by using the steel sheet after annealing had been performedas a raw material steel sheet, by performing three kinds of quenchingtreatments as described below, and by investigating the hardness(quenched hardness) of the steel sheet after quenching had beenperformed, hardenability was evaluated. The results are given in Table2.

By taking a flat-sheet-type test piece (having a width of 15 mm, alength of 40 mm, and a thickness of 4 mm) from the central portion inthe width direction of the steel sheet (raw material steel sheet) afterannealing had been performed, a quenching treatment was performed on theflat-sheet-type test piece by using a method in which cooling (watercooling) was performed with water immediately after the test piece hadbeen held at a temperature of 870° C. for 30 seconds or a method inwhich cooling (120° C.-oil cooling) was performed with oil having atemperature of 120° C. immediately after the test piece had been held ata temperature of 870° C. for 30 seconds. By measuring the hardness atfive points in the cut surface of the test piece which had beensubjected to the quenching treatment by using a Vickers hardness meterwith a load of 1 kgf, and by deriving an average hardness, quenchedhardness was defined as the average hardness.

In addition, by taking a disc-type test piece (having a diameter of 55mmφ and a thickness of 4 mm) from the central portion in the widthdirection of the steel sheet (raw material steel sheet) after annealinghad been performed, a quenching treatment was also performed by using aninduction hardening method (heating the test piece to a temperature of1000° C. at a heating rate of 200° C./s and then cooling the test piecewith water). At this time, by measuring the hardness at two points inthe cut surface of the test piece at the outermost periphery of the testpiece by using a Vickers hardness meter with a load of 0.2 kgf, and byderiving an average hardness, quenched hardness was defined as theaverage hardness.

A case where all of the criteria for satisfactory quenched hardnessgiven in Table 3 in the case of water cooling after holding at atemperature of 870° C. for 30 seconds, in the case of 120° C.-oilcooling after holding at a temperature of 870° C. for 30 seconds, and inthe case of induction hardening were satisfied was judged assatisfactory (O), that is, the case of excellent hardenability. A casewhere one of the criteria for satisfactory quenched hardness given inTable 3 in the case of water cooling after holding at a temperature of870° C. for 30 seconds, in the case of 120° C.-oil cooling after holdingat a temperature of 870° C. for 30 seconds, and in the case of watercooling in induction hardening was not satisfied was judged asunsatisfactory (x), that is, the case of poor hardenability. Here, Table3 indicates the empirical values of quenched hardness corresponding tosufficient hardenability in accordance with C content.

As Table 2 indicates, it is clarified that the hot-rolled steel sheetsof the examples of the present invention had a microstructure whichincluded ferrite and cementite and in which the number density ofcementite grains in the ferrite grains was 0.08 pieces/μm² or less, ahardness of 73 or less in terms of HRB, and a total elongation of 39% ormore, which means these hot-rolled steel sheets were excellent in termsof cold workability and hardenability.

TABLE 1 Ac₁ Ar₃ Transfor- Transfor- mation mation Chemical Composition(mass %) Temper- Temper- Steel Sb, Sn, Bi, ature ature Code C Si Mn P Ssol. Al N B Ge, Te, Se Other (° C.) (° C.) Note A 0.35 0.02 0.36 0.010.003 0.038 0.0035 0.0032 Sb: 0.009 — 719 801 Within Scope of InventionB 0.36 0.01 0.34 0.01 0.003 0.033 0.0038 0.0028 Sb + Sn: 0.015 — 719 797Within Scope of Invention C 0.35 0.01 0.35 0.01 0.003 0.035 0.00390.0019 Sb: 0.010 — 719 799 Within Scope of Invention D 0.20 0.01 0.390.01 0.003 0.033 0.0036 0.0035 Sb: 0.010 Cr: 0.21 719 827 Within Scopeof Invention E 0.38 0.02 0.34 0.01 0.003 0.038 0.0038 0.0025 Sb + Ge +Mo: 0.02 719 796 Within Scope Te + Se: 0.010 of Invention F 0.40 0.010.33 0.01 0.003 0.042 0.0040 0.0015 Sb + Bi: 0.015 Ni: 0.05 719 794Within Scope of Invention G 0.28 0.01 0.40 0.01 0.003 0.037 0.00350.0038 Sb: 0.009 Cr: 0.22 719 811 Within Scope of Invention H 0.35 0.010.36 0.01 0.003 0.038 0.0038 0.0028 Sb + Sn + — 719 800 Comparative Bi +Ge + Example Te + Se: 0.001

TABLE 2 Micro- Microstructure of Hot-rolled structure of and AnnealedSteel Sheet Hot-rolled Spheroidizing Average Hot Rolling Condition SteelSheet Annealing Cementite Finishing Volume Condition Cementite AverageGrain Average Delivery Coiling Fraction Annealing Density CementiteDiameter Ferrite Sam- Temper- Cooling Temper- of Pro- Temper- in FerriteGrain in Ferrite Grain ple Steel ature Rate ature eutectoid atureAnnealing Grain Diameter Grain Diameter No. Code (° C.) (° C./s) (° C.)Ferrite (%) (° C.) Time (h) Phase (piece/μm²) (μm) (μm) (μm) 1 A 860 80620 31 715 30 Ferrite + 0.05 0.80 0.58 7 Cementite 2 A 870 40 610 20 71530 Ferrite + 0.07 0.77 0.55 8 Cementite 3 A 860 29 620 9 715 30Ferrite + 0.08 0.80 0.60 9 Cementite 4 A 870 10 620 3 715 30 Ferrite +0.10 0.58 0.37 10 Cementite 5 A 870 160 620 3 715 30 Ferrite + 0.10 0.570.35 6 Cementite 6 A 870 50 750 15 720 50 Ferrite + 0.03 1.40 0.70 14Cementite 7 B 850 50 580 15 715 30 Ferrite + 0.07 0.78 0.50 9 Cementite8 B 830 90 630 25 710 25 Ferrite + 0.08 0.73 0.53 9 Cementite 9 B 950 50580 4 715 30 Ferrite + 0.14 0.50 0.30 10 Cementite 10 B 830 125 630 8710 25 Ferrite + 0.08 0.65 0.48 9 Cementite 11 C 850 80 590 27 715 30Ferrite + 0.06 0.78 0.47 8 Cementite 12 C 850 32 590 13 715 30 Ferrite +0.07 0.77 0.50 9 Cementite 13 D 870 75 600 28 715 30 Ferrite + 0.07 0.740.55 12 Cementite 14 E 830 50 650 20 715 30 Ferrite + 0.06 0.72 0.51 9Cementite 15 F 820 80 630 23 715 30 Ferrite + 0.06 0.94 0.80 8 Cementite16 G 870 80 610 29 715 30 Ferrite + 0.07 0.95 0.81 11 Cementite 17 H 86050 610 20 715 30 Ferrite + 0.07 0.75 0.55 10 Cementite Difference InAverage N Concentration Property of between Hot-rolled Region (Contentand Annealed within 150 μm of Solid Hardness of Quenched Steel Sheet ofSurface Content Solution B)/ Steel Sheet (HV)* Sam- Hard- Elon- Layerand of Solid (Total B 120° C.- Evaluation Evaluation ple ness gationSteel Sheet Solution B Content) × Water Oil Induction of Cold of Harden-No. (HRB) (%) (mass ppm) (mass %) 100 (%) Cooling Cooling hardeningWorkability ability* Note 1 63 43 20 0.0026 81 610 552 605 ∘ ∘ Example 267 41 20 0.0025 78 603 548 597 ∘ ∘ Example 3 73 39 20 0.0026 81 610 552600 ∘ ∘ Example 4 75 38 20 0.0025 78 604 547 598 x ∘ Comparative Example5 75 38 20 0.0025 78 615 555 610 x ∘ Comparative Example 6 60 45 200.0021 66 610 540 550 ∘ x Comparative Example 7 69 40 20 0.0023 82 615560 607 ∘ ∘ Example 8 72 40 20 0.0024 86 618 563 610 ∘ ∘ Example 9 78 3620 0.0024 86 605 540 603 x ∘ Comparative Example 10 73 39 20 0.0024 86619 562 614 ∘ ∘ Example 11 64 41 20 0.0016 84 605 558 600 ∘ ∘ Example 1272 40 20 0.0016 84 604 556 599 ∘ ∘ Example 13 60 45 20 0.0031 89 452 370447 ∘ ∘ Example 14 69 40 20 0.0019 76 620 560 610 ∘ ∘ Example 15 71 3920 0.0011 73 624 565 618 ∘ ∘ Example 16 66 42 20 0.0031 82 560 470 550 ∘∘ Example 17 68 40 190  0.0004 14 605 410 570 ∘ x Comparative Example*refer to Table 3 for information about quenching condition and theevaluation criteria for satisfactory hardenability

TABLE 3 Hardness Hardness Hardness of Sample of Sample of SampleWater-cooled 120° C.-oil-cooled Water-cooled in after Holding afterHolding Induction C content at 870° C. at 870° C. Hardening (mass %) for30 s (HV) for 30 s (HV) (HV) 0.20 or more and ≧440 ≧360 ≧435 less than0.35 0.35 or more and ≧600 ≧530 ≧595 less than 0.38 0.38 or more and≧610 ≧540 ≧605 less than 0.40 0.40 ≧620 ≧550 ≧615

1. A high-carbon hot-rolled steel sheet having a chemicalcomposition-containing, by mass %, comprising: C: 0.20% or more and0.40% or less, by mass %; Si: 0.10% or less, by mass %; Mn: 0.50% orless, by mass %; P: 0.03% or less, by mass %; S: 0.010% or less, by mass%; sol.Al: 0.10% or less, by mass %:, N: 0.0050% or less, by mass %; B:0.0005% or more and 0.0050% or less, by mass %; at least one of Sb, Sn,Bi, Ge, Te, and Se in an amount of 0.002% or more and 0.030% or less intotal, by mass %; and Fe and inevitable impurities, wherein: aproportion of a content of a solid solution B to a content of B is 70%or more, the steel sheet has a microstructure including ferrite andcementite, the density of cementite in ferrite grains is 0.08 pieces/μm²or less, the steel sheet has a hardness of 73 or less in terms of HRB,and the steel sheet has a total elongation of 39% or more.
 2. Thehigh-carbon hot-rolled steel sheet according to claim 1, wherein thechemical composition of the steel sheet further comprises at least oneof Ni, Cr, and Mo in an amount of 0.50% or less in total, by mass %. 3.The high-carbon hot-rolled steel sheet according to claim 1, wherein: anaverage grain diameter of all the cementite in the steel sheet is 0.60μm or more and 1.00 μm or less, and an average grain diameter of thecementite in the ferrite grains is 0.40 μm or more.
 4. A method formanufacturing a high-carbon hot-rolled steel sheet, the methodcomprising: performing hot rough rolling on steel, the steel having achemical composition comprising: C: 0.20% or more and 0.40% or less, bymass %. Si: 0.10% or less, bv mass %, Mn: 0.50% or less, by mass %, P:0.03% or less, by mass %, S: 0.010% or less, by mass %, sol.Al: 0.10% orless, by mass %, N: 0.0050% or B: 0.0005% or more and 0.0050% or less,by mass %, at least one of Sb, Sn, Bi, Ge, Te, and Se in an amount of0.002% or more and 0.030% or less in total, by mass %, and andinevitable impurities, then, after the hot rough rolling, performingfinish rolling with a finishing delivery temperature equal to or higherthan the Ar_(a) transformation temperature and 870° C. or lower:, then,after the finish rolling, cooling the hot-rolled steel sheet to atemperature of 700° C. at an average cooling rate of 25° C./s or moreand 150° C./s or less; then, after the cooling, coiling the cooled steelsheet at a coiling temperature of 500° C. or higher and 700° C. or lowerin order to obtain a steel sheet having a microstructure includingpearlite and, in terms of volume fraction, 5% or more of pro-eutectoidferrite: and then, after the coiling, annealing the steel sheet at atemperature equal to or lower than the Ac₁ transformation temperature.5. The method for manufacturing a high-carbon hot-rolled steel sheetaccording to claim 4, wherein the chemical composition of the steelfurther comprises at least one of Ni, Cr, and Mo in an amount of 0.50%or less in total. total, by mass %.
 6. The high-carbon hot-rolled steelsheet according to claim 1, wherein the density of the cementite in theferrite grains is 0.06 pieces/μm² or less,
 7. The high-carbon hot-rolledsteel sheet according to claim 1, wherein the microstructure of thesteel sheet further includes pearlite and, in terms of volume fraction,5% or more of pro-eutectoid ferrite.
 8. The method for manufacturing ahigh-carbon hot-rolled steel sheet according to claim 4, wherein: thesteel sheet has a microstructure including ferrite and cementite, andthe density of cementite in ferrite grains is 0.08 pieces/μm² or less.9. The method for manufacturing a high-carbon hot-rolled steel sheetaccording to claim 8, wherein the density of the cementite in theferrite grains is 0.06 pieces/μm² or less.
 10. The method formanufacturing a high-carbon hot-rolled steel sheet according to claim 4,wherein the steel sheet has a hardness of 73 or less in terms of HRB.11. The method for manufacturing a high-carbon hot-rolled steel sheetaccording to claim 4, wherein the steel sheet has a total elongation of39% or more.